Uplink multi-user access communication can be implemented by different multiple access technologies, such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA) and Space Division Multiple Access (SDMA). The Code Division Multiple Access (CDMA) technology, which is one very important category that implements the uplink multi-user access communication, can provide an excellent access performance, and has been adopted by a number of wireless communication standards.
For an access process using CDMA, firstly a plurality of access terminals perform spreading processing on data symbol, obtained after amplitude and phase modulation (such as Quadrature Amplitude Modulation (QAM)) is performed on data to be sent, by using a spreading sequence with a certain length (such as a spreading sequence with a length of L formed by L data symbol or L data elements), respectively. The spreading processing refers to a process in which each modulated data symbol is multiplied by each symbol of the spreading sequence to form a symbol sequence with the same length as that of the used spreading sequence. Specifically, each modulated data symbol (such as a corresponding constellation point symbol obtained after QAM is performed on data to be sent) is multiplied by each symbol of the spreading sequence with the length of L, such that each modulated data symbol is spread to be a symbol sequence with the same length as that of the used spreading sequence, i.e., each modulated data symbol will be spread to be L symbols, which is equivalent to that each modulated data symbol is carried by the spreading sequence of the length L. Then, the symbol sequence, obtained after spreading processing, of the plurality of access terminals may be sent on the same time-frequency resources. Finally, a base station receives signals superimposing the spread signals of the plurality of access terminals together after wireless propagation, and separates useful information of each of the terminals from the received superimposed signals through the multi-user receiving detection technology.
The CDMA falls into the category of spread spectrum communication. Since the modulated data symbol of the terminal will be spread to be L symbols after spreading processing is performed on the modulated data symbol of the terminal by using the spreading sequence of the length L, if transmission time of the L symbols after the spreading processing is required to be equal to transmission time of the data symbol before the spreading, then a bandwidth required to transmit the L symbols after the spreading processing needs to be spread by L times, thus the spreading sequence is often referred as a spread spectrum sequence.
The symbols obtained after the spreading processing by the access terminals may be transmitted in the multi-carrier technology (such as Orthogonal Frequency Division Multiplexing (OFDM) and Filter-Bank Multi-Carrier (FBMC)). The combination of the code division multiple access and the multi-carrier technology is the Multi-Carrier Code Division Multiple Access (MC-CDMA) technology.
In the CDMA technology, a spreading processing process of a transmitter is relatively simple: each modulated data symbol is multiplied by each symbol of the spreading sequence with the length L to obtain the L spreading-processed symbols, and then the L symbols are transmitted through the single-carrier technology or multi-carrier technology. However, a receiving process of a receiver of a base station is relatively complicated. How the receiver of the base station separates accurately useful data information of each terminal from the superimposed signals to ensure the multiple access performance of the CDMA system is the key of the CDMA system, which involves two aspects, i.e., the spreading sequence and the receiver. The selection of the spreading sequence is the basis of the performance, and the design of the receiver is the guarantee of the performance.
Specifically, to obtain an excellent multiple access performance, excellent cross-correlation characteristics are required between spreading sequences used by different terminals. If the single carrier code division multiplexing technology is used, the spreading sequences used by the terminals are required to have better auto-correlation characteristics to resist the impact of multipath delay spread. While the multi-carrier code division multiplexing technology can resist the impact of multipath delay spread by means of the multi-carrier technology, and the design of the spreading sequence of the multi-carrier code division multiplexing technology may consider emphatically the cross-correlation characteristics facilitating the receiver to separate multi-user information.
On the basis of the design of the spreading sequence, the base station can use the multi-user receiving detection technology with a high performance to separate the multi-user information to obtain the excellent multiple access performance, such as a successive interference cancellation (SIC) receiving detection technology. However, the complexity of the multi-user receiving detection technology with a high performance is relatively higher.
The selection and design of the spreading sequence are important aspects of the CDMA technology. A Direct Sequence-Code Division Multiple Access (DS-CDMA) technology as a common CDMA technology has been used as an uplink multi-user access technology of a plurality of wireless communication standards and systems. The spreading sequence of the DS-CDMA technology uses a simple binary pseudo-noise (PN) real number sequence. Moreover, the DS-CDMA based on the binary PN real number sequence is also applied to the MC-CDMA technology. The binary pseudo-noise real number sequence may also be referred as a binary pseudo-noise sequence, and a value of each element or symbol of the binary pseudo-noise real number sequence is usually denoted as 0 or 1. The binary pseudo-noise real number sequence can be further represented as a bipolar sequence, that is, 0 is denoted as +1, 1 is denoted as −1, or, 0 is denoted as −1, 1 is denoted as +1.
The design of the spreading sequence further needs to consider the length of the spreading sequence. The longer the spreading sequences are, the more easily the low cross-correlation characteristics between the spreading sequences used by different terminals can be guaranteed, and the more easily more sequences with the low cross-correlation characteristics can be selected, therefore simultaneous access of more terminals can be supported. If the number of the simultaneously accessed terminals is greater than the length of the spreading sequences, then it is considered that the system is in an overload state.
Supporting simultaneous access of a large number of users to the system for communication is an important demand for future wireless communications, which can be implemented by designing a multi-user access communication system based on the code division multiple access with a better overload capacity.
From the perspective of multi-user information theory, greater system capacity or marginal throughput can be obtained by using a non-orthogonal multiple access mode than an orthogonal multiple access mode on the uplink. Therefore, to provide a flexible system design and support simultaneous access of more users, different access terminals may use non-orthogonal spreading sequences. Since the spreading sequences of different access terminals are not mutually orthogonal, the receiving detection performance of each of the access terminals will get worse as the number of simultaneously accessed terminals increases, and interference among multiple users will become more serious as the system is overloaded.
The length of the spreading sequence based on a binary pseudo-noise real number sequence, which is used by the current code division multiple access technology, is relatively longer. When a larger number of user terminals access the system or when the system is overloaded, by using a traditional receiver (such as a Rake receiver) the performance is poor, while by using an interference cancellation receiver (such as an SIC receiver) the receiving detection complexity is very high and the delay is also very great. If binary pseudo-noise real number sequences with a shorter length is used, then the low cross-correlation between the sequences cannot be guaranteed, and serious interference among multiple users will occur when a large number of user terminals access the system or the system is overloaded, which would further influence the multi-user receiving detection performance and the multi-user access communication performance.